Brettanomyces
or Dekkera is any one of nine different species of naturally-occurring
yeast (including B. intermidious, B. lambicus, B. bruxellensis, D.
bruxulensis, etc.) that can impart distinct sensory character to
wine. Europeans refer to this fairly common phenomenon as Dekkera.

Some wine
drinkers consider its presence a flaw, while some consider it an enhancement,
depending upon the severity or concentration, as well as their level of sensory experience
and taste preferences. It is quite a controversial phenomenon
and the mere mention of "Brett" can bring lively and impassioned
discussion to gatherings of wine drinkers.

Contributing
to the controversy is Brett's potential for complexity. In very low concentrations
it may be difficult to identify and can add aromatic notes that can be
savory, smoky, or spicy. An increased level can be very unpleasant, even
noxious and overpower any sense of freshness or fruitiness. This unpleasantness
may be variously described as smelling like antiseptic, Band-Aid, rodent
cage, sweaty horse, wet dog, rancid, or such. Four separate byproducts
(esterases, tetrahydropyradines, volatile fatty acids, and
volatile phenols) contribute to these aromas. Character and intensity
may depend upon the relative concentration and mix of these compounds.

Brettanomyces
can populate many winery sources, especially hard-to-clean areas, such
as cooperage, must lines, and hoses. A favorite environment is oak barrels,
especially if filled with wine that has high pH (3.6+) but low free sulfur
levels and that contains trace amounts of residual sugar and where the
barrels are stored in relatively warm conditions (above 68° F). High
levels of ripeness and sugar content, combined with "minimalist"
winemaking techniques may be factors contributing to a seeming rise in
Brett levels in California wines.

Although
old barrels are almost mythologically blamed, there are indications that
new barrels are much more susceptible to Brett infection. Barrel age and
hygiene, however, seem to be less essential to preventing Brett than more
interventionist winemaking procedures, such as sterile filtering. Targeted
use of sulfur (large additions at well-timed intervals) and diligent temperature
control, along with conscientious equipment cleaning can prevent Brett
from becoming established.

Because it
favors an environment low in carbon dioxide and high in oxygen, wine stored
in barrels following fermentation is the most susceptible, especially
if the environment is warmed to promote malolactic fermentation. Brett seems also to have a "stealth mode" where it is present in wine but undetected and lying in wait until conditions are right for it to reveal its smelly presence.

Although
it is slow-growing and easily dominated by other yeast species, once it
takes hold, Brettanomyces can be very difficult to control. Brett can survive on ethanol, malic acid, some amino acids, as well as much higher and much lower levels of residual sugar, most conditions ineffective to supporting normal wine yeasts. This tenaciousness often
infects wineries insidiously, living in empty used barrels and infecting
other barrels during racking or topping operations, gradually colonizing
the entire winery and growing in flavor influence from imperceptible to
dominant.

Brett
character is almost exclusively found in red wines, since acids necessary
to form some of the indicative aromas are exclusively extracted from grape
skins. Certain grape varieties (Mourvédre) have naturally higher
concentrations of these precursor acids. Some famous
and expensive wines have such predominant Brett aroma that this has become
almost a trademark of house style, a trait of brand recognition. "If
you can't beat 'em, join 'em" may be the unspoken justification for
defenders of Brett character in these wines. However, it is misleading
to attribute these aromas to be indicative of either vineyard or terroir.